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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    The answer to this question is probably be obvious, but I can't find the answer in the manual. If anyone else has this MVHR unit (or a similar one) I'd be grateful if you could set me straight!

    We've just moved into our new build and turned on the MVHR unit for the first time. It sent our electricity consumption through the roof (about 24Kwh per day) and I have worked out that the in line heater was turning itself on. I've currently disabled the in line heater.

    The control panel is very simple. We've got it set to "Summer Bypass Normal". I can see an "indoor" and "outdoor" temp on the control box. Is the "indoor" value effectively a pre-heat target for the in line heater? What does the "outdoor" temp do?

    I've had a look in the manual but couldn't see anything useful.

    Many thanks for any helpful comments,
    • CommentAuthorsnyggapa
    • CommentTimeJul 13th 2016
    I have a same unit but no inline heater - and in mine the indoor and outdoor temps are present but only related to the summer bypass , so control at what point it stops trying to recover heat and just acts as a regular ventillation system.

    How is the inline heater connected to the MVHR , or is it controlled from elsewhere?

    Thanks for the response, I will give the installer a call and get a few details from them about the inline heater.

    Do you know why there is both an indoor and outdoor temp for the summer bypass and how to configure them in relation to each other (I think my unit had defaulted to 22 degrees indoor and 15 degrees out door)?

    Would it be if the internal air is above 22 degrees AND the external air is above 15 degrees, don't preheat. Otherwise do?
    • CommentAuthorsnyggapa
    • CommentTimeJul 13th 2016
    I would guess the inline heater is controlled from somewhere else and not the sentinel unit - maybe a traditional thermostat in the house somewhere? - so these temps you set here will not affect the pre-heater at all, just the opening of the summer bypass ( which can act as a cooler if the outside air is cooler than indoors).

    I can't remember what the temperatures actually "do" but I do remember that it wasn't intuitive!
    • CommentAuthorFlubba
    • CommentTimeJul 13th 2016 edited
    My understanding and logic would suggest the following...

    IF indoor > preset AND outdoor > preset open the summer bypass ELSE do nothing.

    If the indoor temperature is higher even for a short period of time you don't want the summer bypass activating when it could be freezing outside therefore pulling in much much colder air while just dumping heat. However if it's summer and the solar gain is overheating the house you want the ventilation therefore the summer bypass opens.
    • CommentAuthorRobL
    • CommentTimeJul 13th 2016
    I have that unit (VH sentinel kinetic plus, no heater), and it works just as Flubba said.

    In the summer, I set the Indoor to 18C, outdoor to 5C. In the winter I crank up the indoor setpoint to 22C, just to be sure it never kicks into bypass, stealing heat. If you feel annoying cold draughts from vents then they suggest increasing the outdoor setting, so it won't bypass really cold air straight into the house. Obviously it will be less effective at cooling then.
    Thanks everyone for the clarifications on how the indoor / outdoor settings should be interpreted.

    The unit hasn't been commissioned yet, when they come to do that I will ask about the heater. I'm beginning to think it was a waste of money installing it.
    • CommentAuthorsnyggapa
    • CommentTimeJul 14th 2016
    actually I wish I had one :) - pretty sure it will be independently controlled rather than from the vent axia unit though.

    set it independently at about 19 degrees (I think it has to be below the vent axia indoor temperature, otherwise your vent axia will throw away the heat that it produces) and leave it well alone. Well, maybe turn it off entirely in summer but it shouldn't come on anyway
    @snyggapa - mine was installed along the duct on the between the unit and the fresh air inlet on the roof. I haven't figured out how it is controlled yet. However, I guess a lot of air would come in below 19 degrees (particularly at night) so it would run nearly all the time. Something like this must have been happening as it was killing my electricity bills!
    • CommentAuthorsnyggapa
    • CommentTimeJul 14th 2016
    "between the unit and the fresh air inlet on the roof."

    hmm, not sure that's where I would put it as it would heat the heat exchanger - so an immediate ~10% loss of energy. I would expect to see it between the vent axia outlet and before the room outlets - however there may be a good reason why it is done this way I just can't think of it.

    Is it connected (electrically) to the vent axia unit in any way - can you see what wires go into it and where do they go?
    • CommentAuthorEd Davies
    • CommentTimeJul 14th 2016 edited
    Posted By: snyggapahmm, not sure that's where I would put it as it would heat the heat exchanger - so an immediate ~10% loss of energy.
    If I understand Richard correctly it's much worse than that, surely? If the exchanger is 90% efficient then I'd expect a 90% loss, roughly.

    E.g., for easy arithmetic assume a balanced airflow, no losses in the ducts, a 90% efficient exchanger with 20 °C indoor air and 0 °C outdoor air and no condensation happening. Air supplied to the rooms will be at 18 °C and exhaust air at 2 °C.

    Now heat the incoming air to 10 °C with an electric heater, prior to the exchanger. We've now got a 90% efficient exchanger with an effective outdoor temperature of 10 °C so the air supplied to the room is at 19 °C but the exhaust air is at 11 °C. So, of that 10 °C rise in the incoming air 1 °C has gone to do something useful and 9 °C is blown straight out again.
    My understanding of a heater that is installed prior to the MVHR unit on the fresh air inlet is that it serves as frost protection for the unit so it should only be set to operate at a temperature of a few degrees above freezing.
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